Antimicrobials in dog‐to‐dog bite wounds: A retrospective study of 1526 dog bite events (1999‐2019)

Abstract Background Although dog‐to‐dog bite wounds (DBW) are common, few studies worldwide have evaluated antimicrobial usage patterns or appropriateness of use. Objectives Report frequency and results of DBW cultures, including antimicrobial susceptibility patterns. Determine the most commonly prescribed antimicrobials and their appropriateness for the treatment of DBW, and if antimicrobial importance is associated with wound severity, clinic type or year. Animals One thousand five hundred twenty‐six dog bite events involving 1436 dogs presenting with DBW from 3 Australian university clinics from 1999 to 2019. Methods Retrospective study. Medical records were reviewed for presenting signs, culture and susceptibility testing, antimicrobial treatment, and outcome. A partial proportional odds model was used to determine if use of higher importance antimicrobials was associated with wound severity, clinic, or year. Results Antimicrobials were prescribed in 88.1% (1344/1526) of DBW. Amoxicillin‐clavulanic acid was prescribed in 73.4% (1121/1526) of dogs, followed by first‐generation cephalosporins, 18.1% (277/1526). Of a total of 1647 antimicrobial prescriptions, underdosing occurred in 13.4% for AMC (220/1647) and 26.1% (81/310) of dogs prescribed first generation cephalosporins. There was an association between the increased use of high‐importance antimicrobials and wound severity (P < .001), antimicrobial polytherapy (P < .001) and year (P < .001). The odds of the clinic with specialists prescribing high‐importance antimicrobials compared to those of medium importance for DBW was 82% less than that of a semi‐rural, mixed and general practice. Culture and susceptibility (C&S) testing was performed in 1.8% of dogs. Conclusion and Clinical Importance Empirical use of amoxicillin‐clavulanic acid was common for DBW. Increasing wound severity was associated with greater use of high‐importance antimicrobials. While C&S testing was rarely performed, routine susceptibility profiles are recommended to optimize antimicrobial stewardship.

In veterinary medicine, prophylactic antimicrobial therapy is widely used in DBW and is considered 1 of the mainstays of treatment. 5 In addition to increased costs and the risk of adverse effects, inappropriate antimicrobial use could contribute to antimicrobial resistance. 5  is recommended, with additional antimicrobials prescribed based on culture and susceptibility (C&S) results. 2,5,12,20 However, a recent study demonstrated that C&S of DBW by veterinarians is rarely performed. 21 Currently there is limited published data on the most cultured organisms from DBW in South East Queensland (SEQ). Data reporting antimicrobial susceptibility patterns for bacteria isolated from SEQ DBW will improve first choice antimicrobial selection, thereby reducing the rate of development of antimicrobial resistance, improve treatment success and outcomes. The aims of this study were to: 1. determine the most common antimicrobials prescribed to treat DBW and if the importance of the antimicrobial prescribed, as defined by the Australian Strategic and Technical Advisory Group of Antimicrobial Resistance (ASTAG), 22 was associated with wound severity, clinic, or year and 2. determine the frequency of antimicrobial underdosing when used to treat DBW. The frequency and results of culture of DBW in SEQ, including antimicrobial susceptibility patterns was also investigated.

| METHODS
A search of electronic records (ERs) of dogs presenting to 3 teaching hospitals from the same university from 1999 to 2019 was performed using the terms: "DBW," "dog fight," dog-fight-wound, "DFW," "dog attack," and "dog bite." Clinic A was an urban, specialist referral/ general practice hospital with a database extending from December 2002 until December 2013. Clinic B was a semirural, specialist referral/general practice hospital with a database extending from October 2011 to present day and Clinic C was a semirural, mixed and general practice clinic with a database extending from December 1999 to the present day. Consultations within 1 month of an identified DBW consultation were also extracted to ensure inclusion of reexamination.
The ERs were manually reviewed for signalment; time from injury to presentation; wound severity; type of antimicrobial prescribed; if a culture sample was taken; organism(s) cultured and their antimicrobial susceptibilities; hospitalization duration (days), complications and death. Electronic records were included in the study if the cause of injury was a known DBW as determined from the examination text field. The ERs were excluded if the injuries were not definitive for a DBW, incomplete signalment recorded or were part of a prospective DBW study; this resulted in the first dataset, "initial dataset" which was used for descriptive statistics. Subsequently, ERs for dogs which had no antimicrobials dispensed (including those who died or were euthanized at the initial consultation), or had incomplete treatment data were excluded, resulting in the second dataset, "analytic dataset" used for inferential statistical analysis ( Figure 1). Animal ethics was approved by the University of Queensland Animal Ethics Committee.
A previously established grading system was used to categorize wound severity using the examination text. 5

| Statistical analysis
For descriptive statistics, variables were summarized in accordance with their distribution and type, with normal variables as mean (SD), nonnormal variables as median (interquartile range) and categorical/binary data as proportion (%). To evaluate representativeness of the model sample, the initial and analytic datasets were assessed for statistically significant differences using t-tests, Wilcoxon rank-sum (Mann-Whitney) test and chisquare test for normal continuous, skewed continuous and categorical variables, respectively, with Bonferroni's correction for multiple comparisons.
A partial proportional odds model was fitted to the analytic dataset given the ordinal nature of the outcome variable antimicrobial importance, and the odds ratios (OR) represented study sample averaged effects with adjustment for clustering at the dog level. For the variables constrained to the proportional odds assumption, the OR were for being above a specified antimicrobial importance level compared with being at or below that importance level, with the assumption that the OR did not depend upon the importance level. The OR reported was the estimated effect of the given variable on the odds of being in the antimicrobial medium-importance or high-importance vs low importance; or of being in high importance vs low or medium-importance levels. Two OR were reported for the unconstrained variables; the OR for being an antimicrobial of low-importance vs high-importance, and medium vs high-importance. Likelihood ratio tests were used for assessing violation of the proportional odds assumption at a 0.1 level of significance.
Explanatory variables constrained to the proportional odds assumption included wound severity (grade 1-5), type of therapy The global proportional odds assumption was evaluated using a Wald test of the partial proportional odds model vs the multinomial logit model, with no evidence the assumption did not hold were excluded due to death or euthanasia at initial consultation (n = 104), no antimicrobials dispensed (n = 101), and incomplete data (n = 27; Figure 1).     (Table 3).

| Antimicrobials
Prescribed dosages for AMC, cephalosporins and enrofloxacin were compared to recommended dosages. 25,26 Oral and parenteral AMC were prescribed at dosages less than 12.5 mg/kg in 13.4% (220/1647) of prescriptions. Cefazolin and cephalexin were both prescribed at dosages less than 22 mg/kg in 26.1% (81/310) of prescriptions and cefovecin were prescribed lower than 8 mg/kg in 7.7% (1/13) of dogs. Enrofloxacin was prescribed at lower than the recommended 5 mg/kg dose in 11.1% (13/117) of dogs.
The median frequency and duration of oral dosing AMC and cefazolin was twice a day for 7 days (Table 4). Cefovecin was usually given as a 1-off dose which has a therapeutic effect for 14 days. The median frequency of oral and parenteral dosing for enrofloxacin was 1 dose every 24 hours and the median duration was 3 days for the parenteral and 7 days for the oral formulation (Table 4).

| Model for association between higher importance antimicrobials and wound severity, clinic, and year
When comparing the initial dataset of all dogs compared to the analytic dataset used for inferential modeling, differences in antimicrobial importance class, number of antimicrobials prescribed and wound severity grades was found (P < .001; Table 2). Neuter status was also significantly different (P < .001); this could be due to an association between neuter status and death in this sample A partial proportional odds model of the analytic dataset was used to estimate the multivariate adjusted OR of prescribing an antimicrobial of higher importance (Table 6). There was an association between wound severity of the DBW and importance of antimicrobial prescribed (P < .001). As the wound severity increased by 1 grade, there was an estimated 97% (95% CI: 42%-173%) increase in the odds of a higher antimicrobial class being prescribed (Table 6 and Figure 2).
The number of antimicrobials (monotherapy vs polytherapy) was associated with the antimicrobial importance class prescribed (P < .001). There was an estimated 18.9 times increase (95% CI: 9.6-37.3) in the odds of an antimicrobial of higher importance being prescribed when polytherapy was used rather than monotherapy (Table 6). There was an association between duration of hospitalization and antimicrobial importance class prescribed (P = .004). For each additional day of hospitalization, there was an estimated 15% (95% CI: 5%-27%) increase in the odds of an antimicrobial of higher importance being prescribed (Table 6). An association between clinic and level of antimicrobial importance prescribed was detected (P < .001). Clinic C dispensed more high-importance rather than medium-importance antimicrobials than Clinic B (P < .001). The estimated odds for Clinic B prescribing highimportance vs medium-importance antimicrobials was 82% lower than Clinic C (95% CI: 59%-92%), after adjusting for other variables, including year category (Table 6).
Differences between year category and the level of antimicrobial importance class prescribed were detected (P < .001 1.8-9.9) higher for the time period 2015-2019 (P = .001; Table 5).
There The most common signalment of DBW cases was middle-aged, medium sized, pure breed desexed female dogs. Previous studies found on average, small breed (<10 kg), entire male dogs presented more commonly for DBW. 1,4,9,12,28,29 Authors suggested hormonal and territorial triggers in entire male dogs coupled with the vulnerability of smaller dogs predisposed to more severe injury necessitating medical attention. 1,4,28,29 In our study, the antimicrobial use analytic data set had more desexed dogs than the initial dataset (P < .001).
Neuter status and deaths were associated, with a greater proportion of entire dogs euthanized or died. Although not previously reported, it might be due to wound severity or entire dogs being deemed more aggressive by owners prompting euthanasia on behavioral grounds.
However, a study investigating dog bites in humans found no associated between neuter status and the likelihood of inflicting a DBW. 30 Similar to other studies, most dogs presented for veterinary attention less than 8 hours post injury. 1,4,5,11,29 Eighty-eight percent of DBW were prescribed antimicrobials.
AMC was the most prescribed antimicrobial at 73%, followed by firstgeneration cephalosporins with 8% of dogs prescribed oral cephalexin at discharge. This was similar to a recent study of thoracic DBW in which AMC and first-generation cephalosporins were prescribed at 71% and 14%, respectively. 9 The BSAVA and AIDAP guidelines recommend AMC as first-line treatment for DBW, however APCAH Studies assessing prescribing of antimicrobial by Australian veterinarians found enrofloxacin was dispensed in 3.2% to 18% of cases. 32,33 In New Zealand, fluoroquinolones were prescribed in 12% of cases and of these, 53% had no C&S testing performed. 34 Similar to our study, an association between the use of fluoroquinolones and antimicrobial polytherapy was found. 34 Clinic C (semirural/mixed practice) had a higher rate of prescribing antimicrobials of higher importance compared to Clinic B (specialist referral/general practice hospital). Although the referral hospital was likely to be treating more severe cases of DBW, specialist veterinarians could adhere to the principles of antimicrobial stewardship more closely because of more extensive training. Alternatively, nonspecialist veterinarians could be more concerned about ensuring a successful outcome after a single consultation due to owner financial limitations.
Australian companion animal veterinarians from regional areas used less antimicrobials compared to major cities, but the use of highimportance antimicrobials was higher in outer regional areas and major cities compared to inner-regional areas. 33 Although there was no difference in the proportion of high-importance antimicrobials prescribed, emergency and referral centers prescribed more antimicrobials than general practice as routine examinations and vaccinations rarely require antimicrobials. 33 Another study of Australian companion animal veterinarians found no differences in the prescrip- respectively received a single parenteral injection with no ongoing course of oral administration. The reasons for not receiving an ongoing course of antimicrobials are unknown, however the use of a single, 1-off short acting antimicrobial could have been associated with surgical prophylaxis in contaminated wounds. Improved education of students and veterinary practitioners is needed.
The main limitations of this study are due to its retrospective nature. Therefore, misclassification bias might exist, such as wound severity grades, as these were determined based on the examination of text data. Missing data from the medical records could have resulted in selection bias, especially in cases where reexamination and follow up records were inaccessible, such as those discharged to a referring veterinarian. Furthermore, the data from the 3 hospitals were not over the same time period, although the time periods overlapped and year category was accounted for in the statistical model.
There is a potential lack of generalizability of the results; although data was obtained from 3 hospitals, 2 semirural and 1 urban, the results might not reflect cohorts in different states or countries. Due to the small number of C&S tests performed, the small number of isolates and changing antimicrobial breakpoints over time, no conclusions could be made on the appropriateness of the guidelines in this study sample. The ASTAG antimicrobial importance ratings were also used over the WHO guidelines. This has a potential to limit generalizability however, the ASTAG ratings were created and revised with an awareness of WHO guidelines. The WHO guidelines also recognize that implementation at the national level requires national considerations.

| CONCLUSIONS
This study found that AMC is the most commonly prescribed antimicrobial for DBW in SEQ. C&S testing is rarely performed, despite this, the increasing use of antimicrobials of high-importance over the years is concerning, especially as published human and veterinary guidelines recommend they be reserved as last-line therapy. More education is needed to ensure great compliance with prescribing guidelines for DBW; ensure appropriate dosing of antimicrobials; minimize use of high-importance antimicrobials and increasing the frequency of C&S testing. No antimicrobials were used in 74% of grade 1 wounds and 32% of grade 2 wounds. Therefore, it is the authors' recommendation that no antimicrobials be used in low grade DBW, however a prospective placebo controlled blinded study would be ideal to confirm this.
Further prospective research is required to determine appropriate empirical antimicrobial therapy for DBW, and to understand further Australia-wide trends for treatment of DBW, and frequency and effects of underdosing of antimicrobials.

ACKNOWLEDGMENT
No funding was received for this study. The authors thank Dain Heffernan for access to the databases.